1. Field of the Invention
The present invention relates to a process and apparatus for abatement of warm melting point vapors from gas streams.
2. Background Art
Manufacturing plants which have cryogenic liquids available for use in plant processes are finding it advantageous to use boiling cryogenic liquids as the heat transfer medium in the recovery of volatile materials from effluent gas streams. Often the volatile materials to be recovered require an excessively low separation temperature or the gas streams from which the volatile materials are to be recovered are generated intermittently and therefore it is not practical to use mechanical refrigeration to provide the heat transfer capability for condensing the volatile materials out of the gas streams.
The heat exchange between the effluent gas stream and the cryogenic liquid may be direct or indirect. The heat exchange method is typically indirect if it is desired to maintain the cryogenic material at pressure and/or free from contamination so the vaporized cryogen can be re-used in other processes.
A commonly used process for indirect heat transfer employs a shell and tube exchanger, wherein a liquid or partially vaporized cryogen flows through one side of the exchanger and the effluent gas flows through the other side of the exchanger. Direct heat exchange between the liquid cryogen and the effluent gas can be accomplished using any method by which the liquid cryogen contacts the effluent gas, such as a downward spray of the liquid cryogen upon an upwardly rising volume of gas. U.K. Pat. No. 1,582,955 to R. W. Watson et al., entitled: CONDENSATION OF THE VAPOR OF A VOLATILE LIQUID, describes processes like those discussed above in more detail.
The following patents and abandoned patent application disclose subject matter related to the present invention: U.S. Pat. No. 4,150,494 to R. D. Rothchild, entitled Methods and Apparatus for Recovering Solvents; U.S. Pat. No. 4,237,700 to R. D. Rothchild, entitled Methods and Apparatus for Providing Refrigeration; U.S. Pat. No. 4,122,684 to M. J. Clarkson, et al., entitled Method For The Recovery of Volatile Liquids; U.S. Pat. No. 4,464,904 to F. N. Steigman, entitled Process for the Transfer Of Refrigeration; U.S. Pat. No. 4,551,981 to R. Banerjee, entitled Heat Exchange Methods and Apparatus; U.S. Pat. No. 3,535,345 to R. B. Eghert, entitled Method of Producing Phthalic Anhydride, and published patent disclosure, Federal Republic of Germany, No. P 24 11 601.6, K. M. Pohl, entitled Process For The Reduction of Emissions During The Storage and Loading of Volatile Liquids And Device For Carrying Out Such Process.
With the exception of U.S. Pat. No. 3,535,345 to R. B. Eghert, the above processes utilize a cryogenic liquid (a liquid with a normal boiling temperature below about -200.degree. F.) as the heat transfer medium in the recovery by condensation of volatile materials from gas streams.
The methods and means disclosed in the above-referenced cryogenic technology are deliberately restricted to avoid the solidification of volatile materials in the incoming gas stream upon defined heat exchange surfaces (which would foul the exchange surfaces). One approach to the fouling problem has been to eliminate the defined heat exchange surfaces by directly contacting the incoming gas stream or its refluxed condensate with vaporizing cryogen. This approach causes loss of cryogen pressure and contamination of the cryogen by constituents of the gas stream. Thus, the vaporized cryogen used in this direct contact approach is typically vented to atmosphere. The non-recoverability of the cryogen for use as a clean gas at pressure adds greatly to process cost and adds to the volume of gas containing volatiles which is vented to atmosphere. The relatively large gas volume of the vaporizing cryogen tends to loft condensate droplets of the volatiles to be recovered out of the condenser, subverting the process objective which is volatile abatement or recovery.
Many of the advantages encountered by direct contact of the vaporizing cryogen with the incoming gas stream (from which volatiles are to be removed) can be avoided by imposing a suitable intermediary heat exchange fluid between the vaporizing cryogen and the incoming gas stream. Several of the references cited above disclose the use of such an intermediary heat exchange fluid. The intermediary fluid can be indirectly cooled by the cryogen to a bulk temperature above its melting point and then may undergo either direct or indirect heat exchange with the incoming gas streams from which volatile components are condensed, at temperatures exceeding the volatile component melting points. This technique avoids fouling of the defined surfaces of any indirect heat exchangers.
When the chilled intermediary fluid is directly contacted with the incoming gas stream, the above references recommend use of a liquid condensate of one or more of the volatile constituents of the incoming gas stream as the chilled fluid. The references also recommend the removal of any materials which freeze at high temperatures (such as moisture at 32.degree. F.) from both the intermediary fluid and the incoming gas stream prior to implementation of the volatiles recovery method, to avoid freezing of such materials and fouling of indirect heat exchange surfaces between the chilled intermediary fluid and the vaporizing cryogen. It has also been suggested that during indirect heat exchange between the intermediary cooling fluid and the vaporizing cryogen, the temperature of the warm end of the heat exchanger (where the intermediary fluid first enters the exchanger) be controlled to be at least 32.degree. F. to prevent freezing out of any moisture at this location in the exchanger.
There are applications wherein volatile abatement (to particularly low concentrations) is required or wherein the gas stream entering the volatiles reduction operation comprises a volatile material which tends to freeze out (solidly) at relatively high temperatures (such as water at 32.degree. F.). Such applications are not adequately addressed by the referenced cryogenic art. Particularly in volatiles abatement situations, the specified concentration of volatiles permissible in the gas stream exiting the volatiles reduction operation requires an exiting gas stream temperature which is below the melting point of one or more of the volatile constituents. Such a process will necessarily form the solid phase of one or more of the incoming volatile constituents.
In addition, many of the cryogenic processes for volatiles reduction require continuous operation. The methods of volatiles reduction from gas streams disclosed in the cited art require at least one of the following: (1) a pretreatment of the gas stream to remove moisture and/or other high melting point volatiles; (2) limitation of the heat exchange temperature within at least a portion of the volatiles recovery operation to temperatures above the warmest melting point of gas stream volatile materials; (3) duplication of heat exchange equipment to permit regeneration (solids removal from) of heat exchange surfaces. Substantial economies both in terms of capital equipment outlay and/or in costs of operation can be achieved by eliminating the need for any of the three above means within the cryogenic process for volatiles removal.